This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/JP00/06148 (not published in English) filed Sep. 8, 2000.
This invention relates to a torque sensor and a torque detecting method and, in particular, to a torque sensor and a torque detecting method which are suitable for an electric power steering apparatus in an automobile or a power-assisted bicycle.
Conventionally, in order to supplement a force to manipulate a steering wheel (hereinbelow, referred to as a wheel), an electric power steering apparatus has been developed. The electric power steering apparatus is an apparatus which detects a torque applied to the steering wheel from a driver and, in response to the torque thus detected, makes an electric motor provided in a steering mechanism generate a driving force for steering assistance. In comparison with a hydraulic power steering apparatus, the above-mentioned apparatus has such characteristics that no engine power is consumed to drive a hydraulic pump and that the controllability is excellent because the supplemental driving force is electrically controlled.
The above-mentioned electric power steering apparatus is required to have a torque sensor simple in structure and capable of carrying out non-contact detection from the viewpoint of reliability and cost. FIG. 1 shows one example of the torque sensor which is applicable for such use.
In the torque sensor of FIG. 1, a torsion bar 11 made of a material variable in magnetic permeability due to strain, that is, a magnetostrictive material is incorporated between an input shaft and an output shaft for input and output of the torque. On the outer periphery of the torsion bar 11, two coils 12 and 13 are placed with a given distance kept therebetween. On the surface of the torsion bar 11, elongated slits 14 inclined at xc2x145xc2x0 with respect to its center axis are formed in correspondence to the two coils 12 and 13.
When the torque is produced between the input shaft and the output shaft, the torque is applied to the torsion bar 11 so that torsional strain is produced in the torsion bar 11. Generally, the torsional strain generated in a solid cylinder or a hollow cylinder is decomposed into elongation strain in a direction of 45xc2x0 with respect to the longitudinal direction of the solid cylinder or the hollow cylinder and compression strain in a direction perpendicular thereto. Consequently, the elongation strain and the compression strain are respectively caused to occur in the directions of the slits 14 formed on the surface of the torsion bar 11. The magnetic permeabilities of the respective portions change in the manner such that one is increased while the other is decreased, and have levels substantially proportional to the torque applied.
As a result, the inductances of the two coils 12 and 13 change in the manner such that one is increased while the other is decreased. Each of these two coils 12 and 13 is connected to a ground terminal through a fixed resistance as an impedance element, and an a.c. voltage having a predetermined frequency is applied between the fixed resistance and the ground terminal. At this time, a terminal voltage of each of the coils 12 and 13 is supplied to a differential amplifying circuit. In this event, the differential amplifying circuit produces an output voltage proportional to the magnitude of the torque applied. Therefore, it is possible to detect the magnitude of the torque according to the above-mentioned output voltage.
Typically, an electric motor including a strong magnet is used in an automobile or a power-assisted bicycle. In order to avoid the influence of a magnetic field generated from the magnet, a robust magnetic shield is required. This brings about a drawback that the structure around the torque sensor becomes complicated.
Further, the electric power steering apparatus requires a steering angle sensor for detecting a steering direction in order to control the steering supplementing force produced by the electric motor by detecting a steering speed and a steering angle simultaneously when the torque applied to the wheel is detected as described above. The above-mentioned steering angle sensorgenerally adopts a method of detecting the N-pole and the S-pole of a multipolar ring-shaped magnet by the use of a Hall device. In order to prevent a magnetic field leaking from the ring-shaped magnet from influencing the above-mentioned magnetostrictive material, a shield for the torque sensor portion is required also. This results in a drawback that the structure becomes complicated.
Therefore, it is an object of the present invention to provide a capacitive torque sensor which does not require a magnetic shield and has a simple structure.
It is another object of the present invention to provide a torque detecting method using the above-mentioned capacitive torque sensor.
According to the present invention, there is provided a capacitive torque sensor including a bar having a circular cross-section, a dielectric layer which is formed on at least a part of an outer peripheral surface of the bar and which is variable in dielectric constant in dependence upon the strain, and a pair of interdigital electrodes facing the dielectric layer and forming a single capacitor, the interdigital electrodes comprising a plurality of linear electrodes inclined at a predetermined inclination angle with respect to a center axis of the bar.
According to the present invention, there is also provided a torque detecting method using a capacitive torque sensor comprising a bar having a circular cross-section, a dielectric layer which is formed on at least a part of an outer peripheral surface of the bar and which is variable in dielectric constant in dependence upon the strain, and an interdigital electrode capacitor facing the dielectric layer and inclined at a predetermined inclination angle with respect to a center axis of the bar, the method comprising the steps of activating a modulating operation in response to a change in capacitance of the interdigital electrode capacitor to produce a modulation frequency signal, and detecting a torque from the modulation frequency signal.